Within the Daily Caller article is a link to the original report.
Going to Mexico to buy arms from a cartel?
Well, if you are going to try to start a armed rebellion, you are going to need more than just one firearm.
A mass buying in the US is a sure way to get yourself noticed, unless you are a PD/military.
Maybe the cartels offer a volume discount?
And I dont think the idea was for the migrants to buy the weapons themselves.